Brake linings are one of the most consumed elements used in vehicles, and have a significant impact on vehicle safety and operability. Factors including brake pad longevity, durability, and weight can dramatically affect environmental and economic costs of vehicle production, operation, and maintenance. Brake linings can also have an additional environmental impact due to particulates released into the environment by wear during use, compounded by the immense number of vehicles in operation daily.
Traditionally, brake systems were made using asbestos-based friction materials. As negative factors associated with the use of asbestos became known, copper-based friction materials gained prominence. However, the favorability of copper-based friction materials has waned due to economic costs imposed by a finite availability of copper as a resource and due to increasing concerns over harmful emission of copper particulates into the environment. Indeed, several state legislatures have proposed or enacted legislation limiting use of copper in brake lining materials due to risks of contaminating aquatic environments with copper.
Brake assemblies are utilized in a wide variety of vehicles, such as cars, trucks, airplanes, bicycles, and motorcycles. FIG. 1 illustrates a side view of a customary brake pad assembly 10 for a motor vehicle, and FIG. 2 illustrates a cross-section view of the brake pad assembly of FIG. 1. The brake pad assembly 10 includes a pair of brake pads 12 positioned on opposite sides of a rotating body such as a brake disk 14 that rotates with a wheel 16. When actuated, such as by an actuator 18, a pushing member such as a caliper 20 pinches the brake disk 14 between the pair of brake pads 12 to apply a compression force resulting in friction that slows rotation of the wheel 16.
FIG. 3 illustrates a perspective view of a brake pad 100. Typically, a brake pad 100 includes a pad of friction material lining 102 attached to a backing plate 104. The backing plate 104 is typically a solid metal plate, and the friction material lining 102 is generally attached to the backing plate 104 via rivets or high-temperature adhesives. Friction material linings are generally composed of relatively soft but tough and heat-resistant materials with a high coefficient of dynamic friction μd, which for typical brake pads, ranges from about 0.35 to about 0.42.
Since the friction material lining 102 is the portion of the braking assembly which converts a vehicle's kinetic energy into heat, it is desirable for the friction material lining 102 to be able to withstand high temperatures without excessive wear. Copper-free friction materials have been proposed. However, while excluding copper may result in friction materials that approach some performance aspects of copper-based friction materials, such friction materials often include undesirable properties, such as a decreased integrity, longevity, thermal tolerance, acoustic properties, or hardness. U.S. Patent Application Publication 2013/0037360 by Chen et al. discloses a substantially copper-free friction material for brake pads that uses steel fibers to match some performance aspects of copper-based friction materials.
Therefore, what is needed is a friction material for brake linings that is copper-free, and that exhibits improved performance relative to copper-based friction materials without sacrificing other desirable or necessary properties of the brake lining.